1. Field of the Invention
The present invention relates to a halftone type phase shift mask blank and a halftone type phase shift mask, and more particularly to a halftone type phase shift mask blank and a halftone type phase shift mask which can be inspected with high precision.
2. Description of the Related Art
Examples of a phase shift mask include a so-called halftone type phase shift mask described in JP-A-4-136854. The halftone type phase shift mask has such a feature that the function of shifting the phase of an exposed light and the function of substantially intercepting the exposed light are shared in a semitranslucent film pattern, resulting in a simple structure. However, in the case in which the halftone type phase shift mask of this kind is repetitively used as a mask (a reticle) of a reducing exposure projector (a stepper), an equivalent phenomenon to a substantial light exposure is generated in a region which should not be exposed originally due to a shift of the light transmitting region of a covering member (an aperture) and the transfer region of the reticle. Consequently, there is a problem in that a pattern defect and other drawbacks are apt to be caused.
Techniques for solving the problem have already been filed by the present applicant (see JP-A-6-282063). A halftone type phase shift mask according to these techniques further comprises a shielding film pattern (a shielding band) containing chromium as a principal component in the outer peripheral portion of a transfer region in addition to a phase shift film (a semitranslucent film) pattern, and produces an advantage that a region which should not be exposed can be reliably shielded by the shielding film pattern also in case of repetitive use as the reticle of a stepper.
As described in JP-A-7-128840, for example, there has also been proposed a halftone type phase shift mask in which a shielding film pattern is formed in a region which does not contribute to the phase shift effect of a semitranslucent film pattern in a transfer region in order to prevent a bad influence based on the light transmitting property of the semitranslucent film pattern also in the transfer region.
In the halftone type phase shift mask, furthermore, there has been known that an unnecessary light intensity peak (side lobe light) appears in the semitranslucent film pattern. In recent years, particularly, the transmittance of the semitranslucent film tends to be increased from the vicinity of 6% which is a conventional mainstream to the vicinity of 9%, and furthermore, to the vicinity of 15%. In that case, the light intensity of the side lobe light is increased so that an influence thereof cannot be disregarded. Under the present circumstances, therefore, there has been increased the necessity of a halftone type phase shift mask having a structure in which at least a phase shift effect over a semitranslucent film pattern in a transfer region is not influenced and a shielding film pattern is formed in a position in which the light intensity of the side lobe light is reduced.
Further, when a pattern transfer is to be carried out by using the halftone type phase shift mask, furthermore, precision in the transfer of a pattern is deteriorated by the generation of a stray light if an exposed light is reflected over the surface of a shielding film pattern. Therefore, there has also been known a halftone type phase shift mask in which a reflection preventing film formed by a material such as CrON, CrO or CrF is provided on the surface of a shielding film (the shielding film pattern) in order to prevent the generation of the stray light such as described in Japanese Patent No. 2,983,020 gazette (page 6, FIG. 1)).
For an exposed light to be utilized in the use of the phase shift mask, an i-ray (a wavelength of 365 nm) or a KrF excimer laser (a wavelength of 248 nm) is a current mainstream and a change into an ArF excimer laser (a wavelength of 193 nm) or an F2 excimer laser (a wavelength of 157 nm) which has a shorter wavelength has been carried out.
Thus, the microfabrication of a pattern can be carried out with a reduction in the wavelength of an exposed light to be used, while it is desired that the mask should be inspected more strictly.
More specifically, in the halftone type phase shift mask of such a type as to comprise the shielding film pattern described above, when a shielding film provided on the phase shift film is subjected to patterning, a pinhole is formed on the shielding film pattern or the shielding film excessively remains so that a defect is generated. For this reason, it has been desired that the defect is reliably detected.
In an inspecting apparatus using a transmitted light for an inspecting light which is utilized for a conventional inspection of a pattern defect (for example, a KLA300 series), however, it is difficult to distinguish and recognize a difference in a transmittance between a phase shift film (a semitranslucent film) and a shielding film. For this reason, it is hard to say that all the defects of the shielding film provided on the phase shift film can be always detected reliably.
On the other hand, examples of a conventional apparatus for mainly inspecting a foreign substance on a mask include an inspecting apparatus using a reflected light for an inspecting light and an inspecting apparatus using a transmitted light and a reflected light for an inspecting light (for example, STARlight manufactured by KLA-Tencor Co., Ltd. in U.S.A.). In the case in which such a foreign substance inspecting apparatus is used, there is a problem in that the excessive defects of a shielding film can be detected only when the reflectances of a phase shift film and the shielding film are different from each other. Furthermore, there is a problem in that a phase shift film pattern and a shielding film pattern are to be distinguished and recognized to carry out an inspection in the case in which the shielding film pattern is formed in a transfer region.
Under the actual circumstances, the wavelength of an exposure wavelength obtained one generation before is used for the wavelength of an inspecting light to be used in the inspecting apparatus of a photomask, and a g ray (a wavelength of 488 [nm]) is a current mainstream and a change into a wavelength of 364 [nm], 266 [nm] or 257 [nm] has been carried out.
In relation to the change in the wavelength of the exposed light, the following problem also arises.
That is to say, the halftone type phase shift mask is to be redesigned to be adapted to a wavelength thereof with the change in the exposure wavelength. In particular, the reflectance of a film is not always constant at all wavelengths. For this reason, it is necessary to redesign a shielding film including a reflection preventing film in order to exhibit an effective reflection preventing function for an exposed light obtained after the change.
It is ideal that the shielding film is to be designed under the condition to satisfy the following equation in order to effectively prevent a reflection:nd=λ/4wherein λ represents a wavelength of an exposed light, n represents a refractive index of a film material at the wavelength λ, and d represents a thickness of the film. In the design of the shielding film, thus, it is very significant that the thickness of the film also has a value in the composition of the film material in addition to the selection of the same composition. In other words, the reflecting characteristic of the shielding film is determined by relating various elements such as the composition and the thickness of the film to each other. It is not always easy to find optimum design conditions in consideration of all the elements.
Moreover, the requirement for the shielding film is not restricted to a reflection preventing function for the exposed light. For example, in a halftone type phase shift mask of such a type as to comprise a shielding film pattern over a semitranslucent film pattern, it is desired that the shielding film should exhibit an effective reflection preventing function for a drawing laser beam in the case in which laser drawing is used when a resist is to be applied onto the shielding film of blanks and a desirable pattern is to be drawn for the resist in a process for manufacturing the halftone type phase shift mask.
The reason is that a resist pattern, and furthermore, a shielding film pattern to be formed by using the resist pattern as a mask and a semitranslucent film pattern provided thereunder cannot be formed with high precision when the drawing laser beam causes a reflection over the surface of the shielding film. A drawing laser beam in a laser drawing apparatus having a wavelength of 365 [nm] has been put to practical use. In the same manner as the exposed light, the laser beam also tends to have a reduced wavelength at present.
As described above, it is not sufficient that the shielding film exhibits the reflection preventing function for the exposed light, and the shielding film is to exhibit a predetermined reflection preventing function for the drawing laser beam. According to circumstances, therefore, the shielding film is to be redesigned every time at least one of the exposure wavelength and the wavelength of the drawing laser beam (hereinafter referred to as a laser drawing wavelength) is changed. It is very hard to find optimum design conditions which satisfy the above equation and consider a balance with the resist drawing wavelength.
Referring to a further problem, next, a finer pattern can be transferred if the wavelength of an exposed light is reduced, and a mask is to be correspondingly inspected more strictly. More specifically, in the halftone type phase shift mask, the shielding film of blanks is subjected to patterning so that a pinhole is formed on a shielding film pattern or the shielding film excessively remains so that a defect is generated in some cases. For this reason, the inspection is carried out in a final stage of the manufacture of the mask and the mask having such a defect is excluded.
There have been known two kinds of apparatuses for carrying out the inspection which are obtained by a rough division.
One of the apparatuses serves to detect the defect of an inspected film pattern based on a transmitted light intensity obtained when an inspecting light is irradiated on the inspected film pattern provided on a transparent substrate (for example, KLA-300 series manufactured by KLA-Tencor Co., Ltd. in U.S.A., for example).
The other apparatus serves to mainly detect a foreign substance stuck to an inspected film based on the intensity of a reflected light obtained by irradiating an inspecting light on an inspected film pattern provided on a transparent substrate or the intensities of both the reflected light and a transmitted light (for example, STARlight manufactured by KLA-Tencor Co., Ltd. in U.S.A.).
The former apparatus can be applied to the inspection of a halftone type phase shift mask which has been disclosed in the Patent Document 1 and cannot be applied to the inspection of a halftone type phase shift mask comprising a shielding film pattern on a semitranslucent film pattern. The reason is as follows. In the apparatus utilizing a transmitted light, the transmittance of a semitranslucent film pattern which does not substantially contribute to an exposure is recognized to be approximately 0% so that a semitranslucent film pattern cannot be distinguished from a shielding film (a transmittance of 0%) formed thereon even if a transparent substrate (a transmittance of 100%) can be distinguished from the semitranslucent film pattern.
For this reason, the latter apparatus is used for the inspection of the halftone type phase shift mask of this kind. In the same apparatus, if at least the reflectances of the semitranslucent film pattern and the shielding film pattern are varied in principle, the defect of the shielding film pattern can be detected. In the shielding film pattern defect inspection of a mask having a predetermined shielding film pattern in a transfer region, furthermore, a difference in the reflectance from the semitranslucent film pattern is indispensable. In the case in which a finer pattern is to be formed with a reduction in the wavelength of an exposed light, however, it can be supposed that a reduction in the discovery rate of a defect or a foreign substance is unavoidable.
For a countermeasure, it can be proposed that both of the reflectances is to be positively make a predetermined difference or more in order to carry out the inspection more strictly. However, the implementation has the following difficulty. More specifically, in that case, the shielding film (the shielding film pattern) is to show a predetermined difference in a reflectance in relation to the semitranslucent film (the semitranslucent film pattern) for an inspecting light while exhibiting the reflection preventing function for at least an exposed light and a drawing laser beam in respect of the problems described above. Furthermore, a reflectance capable of detecting an inspecting light has an upper limit in an inspection using a reflection, and is usually 40% or less and preferably 30% or less. Consequently, the difficulty of the design of the shielding film is caused to become more serious.
In addition, the wavelength of an inspecting light (hereinafter referred to as an inspection wavelength) also tends to be changed with a change in an exposure wavelength. More specifically, there is a tendency that an inspecting light having the same wavelength as that of an exposed light obtained one generation before is employed. More specifically, a g ray (a wavelength of 488 [nm]) is a current mainstream for an inspecting light. At present, however, a change into an inspection wavelength of 364 [nm], 266 [nm] and 257 [nm] has been carried out. According to circumstances, consequently, the shielding film is to be redesigned with the change in the inspection wavelength. Thus, it is absolutely impossible to endure the troubles.
For the foreign substance inspection of a photomask blank, furthermore, an inspection using a reflected light is carried out. At present, an inspecting light having a wavelength of 488 [nm] is used and it is necessary to correspond to the same wavelength. While the shielding film pattern is generally used as the etching mask of the semitranslucent film, moreover, a reduction in the thickness of the shielding film is also required for forming a shielding film pattern with a high resolution with the microfabrication of a pattern in recent years. Thus, it is also necessary to take the thickness of the shielding film into consideration.